I did a pregnancy test when my period was already a week late, and it was negative

HI Medic,

I had sex with my boyfriend on the 16th day of my period without any protection or it was ejaculated inside so, I took 2 ipill after 58 hours to make it more effective because I was worried and thought only one tablet might be less.Normally my period cycle is 8 days but it's already 10 days passed than my period date and I did pregnancy test after a week then my period should come and it was negative but my period is delayed for 12 days already.How longest time it might take for my period?and can I really be sure that I won't be pregnent suddenly?